[Discuss-gnuradio] Gnuradio clock coherence?

[John Ackermann N8UR]

What is the clock source for a Gnuradio "internal" source such as the 
Signal Source block?  If there is no hardware device connected, is the 
block ultimately driven by the PC clock?

Then, if there is a hardware source (SDR) as well as an internal source, 
does the hardware source provide the master clock for the flowgraph, 
such that any other software-derived sources are coherent with it?  Or 
does a Signal Source block still get its clock from the PC?

My real question is whether it's safe to assume that a flowgraph is 
entirely clocked by the samples from its hardware source, and 
consequently that processing within the flowgraph (e.g., oscillator and 
mixer for tuning) does not introduce potential frequency errors.

Thanks,
John